The present invention is directed to a method for producing an adherent copper coating on a zinc or zinc alloy article without the use of cyanide as a complexor in the copper plating bath.
Zinc and zinc alloy products are generally produced as zinc die castings or formed from sheet using rolled zinc alloys. To enhance the decorative features of articles made from zinc alloys and to enhance the corrosion resistance properties of such articles, it is common in the industry to coat zinc or zinc alloy articles with other metals. This coating is generally accomplished by electroplating metals, such as copper, nickel, chromium, tin, and brass, onto the surface of the zinc or zinc alloy product. Copper coated zinc alloy is advantageously employed to produce various cast, extruded, molded articles, and the like, including trims, fixtures, and coin blanks, such as zinc alloy (2% copper) penny blanks.
Zinc and zinc alloy articles have traditionally been electroplated with copper using cyanide as a component of the copper electroplating process. Because of the toxicity of cyanide, efforts have been made to replace the copper cyanide plating solutions with other plating solutions that do not contain cyanide.
There are three general types of copper plating processes that are currently used. The first process is a cyanide or non-cyanide alkaline bath, which may contain cyanide or not. A second type of process uses an acid bath, and contains sulfate, or alternatively, fluoborate, as a complexor. The third type of process is a mildly alkaline pyrophosphate complexed bath.
While it would be desirable to use copper pyrophosphate plating baths in place of the more toxic cyanide plating baths for electroplating on zinc and zinc alloys, prior efforts to this regard have produced non-functional loosely adherent immersion copper deposits.
Copper pyrophosphate electroplating baths have been used, for example, in metallizing magnetic ceramic materials, such as inductors and transformers. U.S. Pat. No. 6,007,758, to Fleming et al., which is hereby incorporated by reference, discloses electroplating copper pyrophosphate onto a conductive material. The reference teaches that the conductive material, which is applied as a conductive ink, contains silver/palladium particles. Copper is electroplated on to the conductive material using a copper pyrophosphate bath, to produce an adherent copper layer on the surface.
U.S. Pat. No. 6,068,938, to Kato et al., which is hereby incorporated by reference, discloses coating a magnesium based article by a zinc immersion coating, wherein the zinc layer has an underlayer of copper plating formed using a copper pyrophosphate solution. This patent indicates that in a conventional zinc immersion process, copper pyrophosphate is not used because a zinc layer with a xe2x80x9cdefective portionxe2x80x9d is produced, which exerts a wrong influence of plate adhesion. In addition, the invention disclosed by this patent does not contemplate plating a zinc or zinc alloy article using a copper pyrophosphate solution.
U.S. Pat. No. 6,054,037, to Martin, which is hereby incorporated by reference, discloses a copper plating solution that does not contain cyanide. The plating solution is an organophosphanate base, cyanide-free copper electrolyte. However, this plating solution does not contain copper pyrophosphate. In addition, there is no indication as to the adherence of the copper plating on the zinc or zinc alloy article.
Therefore, a need exists in the art for a process that does not contain cyanide in the plating bath and that produces a strongly adherent copper coating on a zinc or zinc alloy article.
The present invention is directed to a method for producing an adherent copper coating on a zinc or zinc alloy article without the use of cyanide as a component of the plating process. The zinc or zinc alloy article is first immersed in an aqueous nickel pyrophosphate solution and is then electroplated with a copper pyrophosphate solution. Nickel pyrophosphate solutions have not previously been used to form a layer suitable for alkaline electroplating. The method produces an adherent copper coating on the zinc or zinc alloy, which coating can be deformed without any loss of the copper coating
It is an object of the present invention to produce an adherent copper coating on a zinc or zinc alloy without the use of cyanide as a complexor of the copper plating bath.
In accordance with the present invention, a method for copper plating a zinc or zinc alloy article is provided, wherein the method comprises the steps of:
a) cleaning and activating the zinc or zinc alloy article;
b) immersing the cleaned and activated zinc or zinc alloy article in an aqueous nickel solution, wherein the nickel solution comprises a source of nickel, a source of pyrophosphate, and a source of hydroxide; and
c) electroplating the zinc or zinc article in a copper pyrophosphate solution to a desired thickness.